For reading data from an optical disc, the optical pickup head is moved in two directions, i.e. a direction perpendicular to the disc face, which is referred to as a focusing direction, and a direction parallel to the disc face, which is referred to as a tracking direction. Meanwhile, the light emitted by a light source such as a laser diode is focused by an object lens of the optical pickup head on the optical disc, and the light reflected by the optical disc is transmitted to a light sensor to extract data. According to the obtained data, focusing errors and tracking errors are generated and referred to for further adjusting the movement of the pickup head in the focusing direction and the tracking direction.
To correct the focusing error, an astigmatic method can be employed for focusing control. For implementing the astigmatic method, the optical sensor of an optical disc drive is required to include four light receiving parts A, B, C and D for respectively receiving the main beam reflected from the disc, as can be seen in any of FIGS. 1(a)˜1(c). As far as a focusing operation is concerned, the focusing error signal FE is substantially a difference between the summation of the overall light intensity received by the receiving parts A and C and the summation of the overall light intensity received by the receiving parts B and D, i.e. (A+C)−(B+D), where A, B, C and D are light intensities received by the regions A, B, C and D, respectively. FIGS. 1(a)˜1(c) illustrate three kinds of focusing results. When the light emitted by the light source is perfectly focused on the desired point, as shown in FIG. 1(b), the overall light intensity received by the receiving parts B and D will be equal to that the overall light intensity received by the receiving parts A and C, i.e. FE=(A+C)−(B+D)=0. In another case shown in FIG. 1(a), the value of (A+C)−(B+D) is minus, which indicates a focusing position above the perfect position. On the other hand, in the case shown in FIG. 1(c), the positive value of (A+C)−(B+D) indicates a focusing position below the perfect position. The relationship between the voltage of the focusing error signal FE and the depth of the focusing position (or the distance of the focusing position from the lens) is illustrated in FIG. 1(d). The astigmatic method is performed in a closed-loop focusing control manner to zero the focusing error FE, thereby locating the perfect focusing position.
Another focusing control method is so-called as a differential astigmatism method. For implementing the differential astigmatic method, the laser light emitted from a light source is diffracted through a diffraction grating to result in two sub-beams beside the main beam, as shown in FIG. 2(a). In this method, three optical sensors are used. The central optical sensor has four light receiving parts A, B, C and D for respectively receiving the main beam reflected from the disc. Whereas, the bilateral optical sensors have four light receiving parts E, F, G and H for respectively receiving the sub-beam reflected from the disc. According to the differential astigmatism method, the focusing error signal DAD_FE is calculated as DAD_FE=[(A+C)−(B+D)]−[(E+G)−(F+H)]×Kb, where A, B, C, D, E, F, G and H are light intensities received by the regions A, B, C, D, E, F, G and H, respectively, and Kb is a gain-adjusting coefficient. The relationship between the voltage of the differential astigmatism focusing error signal DAD_FE and the depth of the focusing position (or the distance of the focusing position from the lens) is illustrated in FIG. 2(b). Same as the astigmatic method, the differential astigmatism focusing control method is performed in a closed-loop control manner to zero the focusing error DAD_FE, thereby locating the perfect focusing position.
A typical DVD disc consists of two 0.6 mm thick substrates bonded together. Data can be recorded into either or both of the substrates. The basic type of DVD disc is the one with single data-recording side and single data-recording layer, which has a capacity of about 4.7 GB. A single-side dual-layer disc allowing data to be recorded at the same side but different depths enlarges the storage capacity up to 8.5 GB. A double-side single-layer disc doubles the capacity of the single-side single layer disc and is about 9.4 GB. In addition, a double-side dual-layer disc is also available, which has a capacity up to 17 GB.
It is understood from the above description that recording data in different layers or depths is able to largely enhance the storage capacity of an optical disc. In order to read data from different layers of the disc, the focusing position of the light beam is required to be changeable between layers. The shift of focusing positions of the light beam between the recording layers is achieved by a layer-jumping operation in the focusing direction. Therefore, layer-jumping control is required for this type of discs in addition to conventional focusing control.
While the plots of FIG. 1(d) and FIG. 2(b) illustrate the focusing error variations with focusing positions when a single-layer disc is read, FIG. 3 and FIG. 4 illustrate the focusing error variations with focusing positions when a dual-layer disc is read. In FIG. 1(d) and FIG. 3, the astigmatism focusing control method is performed, and in FIG. 2(b) and FIG. 4, the differential astigmatism focusing control method is performed. Compared to the focusing error FE realized by the astigmatism method, the focusing error DAD_FE realized by the differential astigmatism method is generally more accurate and stable than the astigmatic method in the closed-loop control of focusing procedure. However, as far as the layer-jumping procedure of the differential astigmatism method in the open-loop control is concerned, there is fluctuation 33 mixed with two principal peak groups 31 and 32, as shown in FIG. 3, due to the effect of sub-beams. The layer jumping may fail because of focusing on the depth associated with the fluctuation 33 instead of the correct target depth associated with the principle peak group 31 or 32. Accordingly, errors would be introduced into subsequent closed-loop control at the appearance of fluctuation 33.